1. Field of the Invention
The present invention relates to an image recording apparatus, and more particularly to an image recording apparatus wherein a laser beam emitted from a laser diode writes an image on a recording medium.
2. Description of Related Art
Conventionally, in an electrophotographic laser printer, a laser beam optical scanning system has been using a laser diode as a light source. A laser beam emitted from the laser diode is a light diffused within a certain angle, and a convergent lens (a collimator lens) is disposed in front of the laser diode so as to change the diffused light into a parallel pencil of rays or a convergent pencil of rays.
Recently, a micro Fresnel lens has been developed. The micro Fresnel lens has a pattern of concentric circles pitched by microns on a sawtooth surface, and zones among the circles are alternatively transparent and opaque. The micro Fresnel lens uses refraction and diffraction. When a parallel pencil of rays enters the micro Fresnel lens, the rays are bent at the alternatively transparent and opaque zones, and the parallel pencil of rays are changed into a convergent pencil of rays. When the Fresnel lens receives a divergent pencil of rays emitted from its focal point, the Fresnel lens changes the divergent pencil of rays into a parallel pencil of rays.
The micro Fresnel lens can be used in the optical scanning system as a collimator lens. However, in this case, when the wavelength of the laser beam emitted from the laser diode changes, the optical scanning system changes its performance, thereby causing defocus. More specifically, since the Fresnel lens uses diffraction, its focal length changes with a change in the wavelength of the laser beam. The size of a beam spot on an image surface (on a surface of a photosensitive drum) changes according to the focal length of the Fresnel lens. Therefore when the wavelength of the laser beam changes, a finished image will be deteriorated.
The wavelength of the laser beam emitted from the laser diode changes according to the temperature of the laser diode. Main factors of changing the temperature of the laser diode is heat radiated from the light emitting portion of the laser diode itself and a change in temperature in the circumstances. Especially when the laser diode is turned on, the laser diode may be so heated that the wavelength of the laser beam emitted therefrom is changed remarkably.